Ammonia is commonly manufactured by reacting nitrogen and hydrogen in a synthesis loop including a compressor, an ammonia synthesis reactor and ammonia condensation and recovery. The unreacted synthesis gas mixture is typically recycled from the ammonia separator to the compressor and back to the reactor. Make-up synthesis gas is continuously added to the synthesis loop to provide fresh hydrogen and nitrogen. Because the synthesis gas contains argon, methane and other inert components, a purge stream is usually taken from the synthesis loop to avoid the excessive buildup of the inerts in the synthesis loop. The purge gas is typically processed in a hydrogen recovery unit, and a hydrogen-enriched stream is recycled to the synthesis loop. In some cases, the purge gas is used directly in the fuel system with or without any additional treatment or hydrogen recovery.
A significant technological advance in the manufacture of ammonia has been the use of a highly active synthesis catalyst comprising a platinum group metal such as ruthenium on a graphite-containing support as described in U.S. Pat. Nos. 4,055,628; 4,122,040; and 4,163,775; all of which are hereby incorporated herein by reference. Also, reactors have been designed to use this more active catalyst, particularly the catalytic reactor bed disclosed in U.S. Pat. No. 5,250,270 which is hereby incorporated herein by reference. Other ammonia synthesis reactors include those disclosed in U.S. Pat. Nos. 4,230,669; 4,696,799; and 4,735,780; and the like.
Ammonia synthesis schemes have also been developed based on the highly active catalyst. In U.S. Pat. No. 4,568,530, stoichiometrically hydrogen-lean synthesis gas is reacted in a synthesis reactor containing the highly active catalyst in the synthesis loop.
In U.S. Pat. No. 4,568,532, an ammonia synthesis reactor based on the highly active catalyst is installed in series in the synthesis loop downstream from a reactor containing the more conventional iron-based synthesis catalyst.
In U.S. Pat. No. 4,568,531, the purge gas removed from the primary synthesis loop is introduced into a second synthesis loop using the more active synthesis catalyst to produce additional ammonia from the purge stream. Another purge stream, significantly reduced in size, is taken from the second synthesis loop to avoid the excessive buildup of inerts. The second synthesis loop, like the primary synthesis loop, employs a recycle compressor to recycle synthesis gas to the active catalyst converters in the second synthesis loop.
It would be very desirable to convert hydrogen and nitrogen in the purge stream from a conventional ammonia synthesis loop into additional ammonia using a once-through reactor which does not require staged cooling and a synthesis gas recycle compressor.